At present, hydrogen gas supply systems have not been developed as a common infrastructure. Therefore, with devices that require hydrogen in large amounts, e.g., fuel cell systems which are being developed and commercialized as distributed power generation apparatuses, it is often the case that a hydrogen generation apparatus is provided individually at the site where the device is installed.
A hydrogen generation apparatus includes a reformer for generating a reformed gas containing hydrogen through a steam reforming reaction between a hydrocarbon-type raw material, such as city gas or LPG (liquefied petroleum), and steam. The components of the reformed gas generated by the reformer are hydrogen, methane, carbon monoxide, carbon dioxide, and steam. Carbon monoxide (CO) contained in the reformed gas has a poisoning effect on the electrode catalyst of the fuel cell. In view of this, a CO remover for removing carbon monoxide from a hydrogen-containing gas is normally provided downstream of the reformer.
Various types of fuel cells have been in use. Currently, the most prevalent for household use is the solid polymer fuel cell. Where a solid polymer fuel cell is used, in order to suppress poisoning of the electrode (anode) catalyst, CO removal is necessary so that the concentration of CO contained in the reformed gas is about 500 ppm or less, and preferably 10 ppm or less.
Therefore, the CO concentration in the reformed gas cannot sufficiently be reduced only by providing an shift converter for removing CO from the reformed gas through a CO shift reaction, as the CO remover of the hydrogen generation apparatus. In view of this, it is preferred that an oxidizer filled with an oxidation catalyst or a methanator filled with a methanation catalyst is provided, as the CO remover, downstream of the shift converter. With an oxidizer, a very small amount of air is added to the reformed gas to selectively oxidize CO, thereby further removing CO contained in the reformed gas after shift convertion. On the other hand, with a methanator, CO in the reformed gas is methanated and thus removed. Thus, the reformed gas whose CO concentration has been reduced through a shift converter and a CO cleaner is supplied to the anode (fuel electrode) of the fuel cell, thereby preventing the poisoning of the electrode catalyst.
However, if an oxidizer is provided as the CO cleaner, it is necessary to introduce air into the reformed gas, which may complicate the configuration of the hydrogen generation apparatus. On the other hand, if a methanator is provided, there is no need to an element for introducing air into the reformed gas, and therefore the apparatus configuration can be made simple.
A fuel cell system including a methanator as the CO cleaner is disclosed in Patent Document No. 1, for example.